This invention relates to a method for the removal of dust deposited on the interior of the contact apparatus employed for the removal of harmful components such as sulfur oxides and nitrogen oxides from waste gas.
Generally, waste gases from stationary sources such as combustion furnaces, incinerating furnaces and sintering furnaces contain various oxides such as V.sub.2 O.sub.5, Na.sub.2 O, Fe.sub.2 O.sub.3, Al.sub.2 O.sub.3, CaO, SiO.sub.2, NiO and SO.sub.3 and carbon and other dust components in significant quantities as well as such harmful components as sulfur oxides and nitrogen oxides. If a waste gas of such nature is passed through a contact apparatus, particularly a packed-bed type contact apparatus, for the removal of such harmful components, then the dust components described above are deposited on the surface of solid materials including the active components in the contact apparatus, such as, for example, adsorbents for sulfur oxides and catalysts for conversion of nitrogen oxides and others into unharmful compounds, and to the inner wall surface of the contact apparatus, thus gradually increasing the pressure drop or degrading the efficiency of removal of harmful components. Thus, it is extremely difficult for the contact apparatus to be effectively operated on a commercial scale.
As a measure for avoiding the undesirable effects of such dust components, there has been suggested a method which comprises subjecting the influent waste gas to treatment with an electrostatic dust precipitator or scrubber for removal of the dust components in advance. This method proves to be expensive in terms of equipment and operation. If the operation happens to involve a scrubbing treatment or some other similar treatment, then the temperature of the waste gas is greatly lowered. The cooled waste gas must then be heated up to at least the temperature it had before dust removal, more specifically to the temperature suitable for removal of harmful components, so as to be effectively treated in the contact apparatus.
There has also been proposed a method for preventing deposition of dust components as much as possible by improving the internal structure of the contact apparatus, namely by providing the interior of the contact apparatus with regularly arranged flow spaces adapted to cause the current of gas to flow in one fixed direction. If such an arrangement is used, the dust components invariably deposit on the wall surfaces defining the flow spaces, after the contact apparatus has been used for a long time, though not so heavily as in the operation of the packed-bed type contact apparatus.
Numerous techniques have so far been proposed for preventing dust components from depositing on the interior of contact apparatus, as described above. However, there has never been developed a technique capable of effectively removing such dust components which have already been deposited on the inside of the contact apparatus. Virtually no study has heretofore been made in search of a technique for effective removal of the dust components of the type suffered to deposit on the interior of contact apparatus while said contact apparatus is being operated under normal conditions in a process for the removal of harmful components contained in a waste gas.